the process variable is directly measured in units that represent the basic nature of that variable
Direct Method
the measurement of a process variable indirectly by using another variable
Inferential Method
*TYPES OF MEASUREMENT
- Single Point Type
* Continuous Type
- the measurement depends on a fixed value of the process variable
- the reading is indicated either as high or low
SINGLE POINT TYPE
the measurement indicates the actual value of the process variable
CONTINUOUS TYPE
refers to the capability of the instrument to measure a variable
INSTRUMENT RANGE
refers to the set of values within the instrument measuring range where the scaled output; 4-20 mA, 3-15 psi or 1-5 V is set during calibration
CALIBRATION RANGE
it is the distance (or difference) between the upper range value (URV) and lower range value (LRV)
INSTRUMENT SPAN
the highest value of the measured process variable that the output of a transmitter is currently configured to measure
UPPER RANGE VALUE
the lowest value of the measured process variable that the analog output of a transmitter is currently configured to measure
LOWER RANGE VALUE
refers to a true-or-false condition
“discrete” variable or measurement
one that is only able to indicate whether the measured variable is above or below a specified setpoint
DISCRETE SENSOR
- typically take the form of “switches”, built to trip when the measured quantity either exceeds or falls below a specified value
- these devices are less sophisticated than so-called continuous sensors capable of reporting an analog value, but they are quite useful in industry
DISCRETE SENSORS
are sometimes referred to in the electrical industry as form-A contacts
Normally-Open Switch
sometimes referred to in the electrical industry as form-B contacts
Normally-Close Switch
- an electrical switch actuated by a person’s hand motion.
- this may take the form of toggle, pushbutton or rotary
HAND SWITCHES
- detects the physical motion of an object by direct contact with that object.
- a limit switch will be in its “normal” status when it is not in contact with anything
LIMIT SWITCH
- detects the proximity (closeness) of an object.
- these switches are non-contact sensors, using magnetic, electric or optical means to sense the proximity of objects
PROXIMITY SWITCHES
- detects the presence of fluid pressure
- use diaphragms or bellows as the pressure sensing elements, the motion of which actuates one or more switch contacts
PRESSURE SWITCH
- detects the level of liquid or solid (granules or powder) in a vessel.
- often use floats as the level-sensing element, the motion of which actuates one or more switch contacts
LEVEL SWITCH
- detects the temperature of an object
- often use bimetallic strips as the temperature-sensing element
TEMPERATURE SWITCH
- detects the flow of some fluid through pipe
- use “paddles” as the flow-sensing element, the motion of which actuates one or more switch contacts
FLOW SWITCH
-the fluid equivalent of an electrical switch: a device that either allows unimpeded flow or acts to prevent flow altogether
ON/OFF VALVE
-a voltage or current whose magnitude represents some physical measurement or control quantity
ANALOG ELECTRONIC SIGNAL
